The present invention relates to negative resistance microwave circuits, and more particularly to parametrically-stable microwave circuits using an IMPATT diode as the active element.
Large signal parametric instabilities in IMPATT diode circuits are discussed in an article by W. E. Schroeder entitled "Spurious Parametric Oscillations in IMPATT Diode Circuits" in the Bell System Technical Journal, Vol. 53, No. 7, Sept. 1974, pages 1187- 1210. According to this article, spurious oscillations are frequently observed in strongly driven IMPATT amplifiers and oscillators. In the case of an amplifier, such spurious oscillations result in a signal frequency at the output not present at the input of such an amplifier. In the case of an oscillator, there is a signal present in addition to the main desired oscillation due to such spurious oscillations. In IMPATT diode circuits, the two most frequently observed spurious oscillations are the parametric-pair type and the subharmonic type instability. In the former type, two spurious signals have frequencies adding up to the desired signal frequency, i.e., the pump signal frequency, while in the latter type a spurious signal occurs at one half the frequency of the pump signal. Accompanying the onset of spurious oscillations, there is often a premature saturation of rf output power, a degradation of the circuit noise performance, and/or a shift in the center frequency of the locking band. In order to suppress parametric-pair oscillations, it is necessary to control the impedance presented to the diode over a very broad frequency range.
One approach used to achieve broadband impedance control in IMPATT diode circuits is described in U.S. Pat. No. 3,534,293 to E. T. Harkless, issued Oct. 13, l970, and assigned to applicants' assignee, wherein a circuit load is coupled to the diode via a high-Q resonant cavity. Away from the resonant frequency of the cavity, the circuit load is decoupled and the diode signals are coupled to a dissipative load. In other words, only energy within the bandwidth of the resonator is transmitted through the resonator to the circuit load, while energy at other frequencies is dissipated in a dissipative impedance. This known technique appears to be limited to microwave circuits wherein a high Q is acceptable. However, in amplifiers and in a locked-oscillator-type amplifier wherein a low Q is mandatory, this known arrangement is not easily applicable. Also, due to the resonant cavity inserted in the circuit, rf power losses in the cavity and in the dissipative impedance are unavoidable.
In U.S. Pat. No. 3,792,375 to C. A. Brackett, issued Feb. 12, 1974, and assigned to applicants' assignee, spurious bias circuit oscillations in IMPATT oscillators are suppressed by inserting in the bias circuit a filter including an inductance shunted by a dissipative resistance thereby providing a low-loss path for the dc bias current. This known arrangement is directed to avoid burnout of negative resistance diodes when operated at moderate power levels. However, this known technique is ineffective for suppressing parametric spurious oscillations in strongly driven IMPATT diode circuits such as amplifiers and oscillators operating at high rf levels.
A broadband frequency steering network connected across the two terminals of an IMPATT diode is described in U.S. Pat. No. 3,836,863 to H. Seidel, issued Sept. 17, 1974, and assigned to applicants' assignee. The frequency steering network comprises an array of n reactive couplers each having a predetermined set of coupling parameters. This known network provides a broadband resistance having a prescribed resistance-frequency characteristic for suppressing spurious oscillations in the bias circuit of IMPATT amplifiers and oscillators, which spurious oscillations are of the type discussed in the above patent to C. A. Brackett. Thus, the thrust of this known arrangement is to couple a direct current bias source to an IMPATT oscillator through an essentially resistanceless path while simultaneously loading the diode with a positive resistance independent of the bias source output resistance.